Mirror structure with at least one component of an optical sensor integrated therewith

ABSTRACT

A mirror structure is provided in which at least a portion of a wavefront sensor is integrated with a mirror. In particular, a mirror structure is provided in which a Hartmann mask or a microlens array of a Shack-Hartmann wavefront sensor is integrated with a mirror to provide a very compact wavefront detector/corrector in a single device. Such a mirror structure may be used in a laser cavity to simplify adaptive optics in the cavity. Furthermore, a Hartmann Mask may be integrated with self deforming mirror comprising an active PZT layer bonded to a passive mirror substrate, wherein the Hartmann Mask comprises an array of apertures formed through the active PZT layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase of PCT/GB2009/051539, filed Nov. 16,2009, which claims priority to British Patent Application No. 0821150.0,filed Nov. 19, 2008 and European Patent Application No. 08275076.1 filedNov. 19, 2008 each of which are incorporated by reference herein intheir entireties.

The present invention relates to mirror structures and in particular,but not exclusively, to a mirror structure in which at least a part ofan optical wavefront sensor is integrated with a mirror. Such devicesare particularly suited to use in adaptive laser optics.

Image sensors are used widely as diagnostic tools for lasers, forexample to check laser beam quality within a test procedure before alaser system is delivered, but they may also be incorporated into alaser system to sample the laser beam with a beam splitter. Used in thisway, the image sensor is limited to providing information on theintensity distribution within the laser beam. An image sensor maycomprise a conventional array of charge-coupled devices (CCDs) arrangedto detect the profile of light intensity across the width of the laserbeam.

A more sophisticated sensor is a standard Shack-Hartmann sensor whichcomprises a microlens array that samples an incident wavefront over theaperture of the sensor and an image sensor that detects the position ofa focussed spot of light in each microlens sub-aperture. Position datafrom the image sensor provides a measurement of the average wavefrontslope across each sub-aperture as represented by the displacement of therespective focussed spot of light from the axis of the respectivemicrolens. In order to provide an accurate representation of the totalwavefront, the size of each sub-aperture is made small enough to capturethe highest order aberration expected. However, the complexity ofsubsequent data processing increases rapidly with the number ofmicrolenses in the array, reducing the sensor's speed. The image sensoris generally configured as a square array of individual sensors, such asa CCD array, but an array of position-sensitive detectors (PSDs) may beused alternatively.

Where the application is not photon-limited, it is possible to replacethe microlens array with a Hartmann mask. A Hartmann mask is generally amechanical mask with a square array of apertures. While the principle ofoperation is the same as for the standard Shack-Hartmann sensor, theHartmann mask will obstruct more of the incident light than a microlensarray.

It is known to use Shack-Hartmann sensors directly as diagnostic toolsfor deriving wavefront and intensity information for a laser, and hencethe intensity profile, as one measure of the quality of a laser beam. Inparticular, it is known to use either or both of a conventional CCDarray and a Shack-Hartmann sensor in conjunction with a wavefrontcorrector or deformable mirror as part of an Adaptive Optics System, forexample in an intra-cavity laser adaptive optics (AO) arrangement.

From a first aspect, the present invention resides in a mirrorstructure, comprising a mirror having a reflective surface arranged toallow a portion of incident light to pass therethrough and wherein themirror has at least one component of an optical sensor integratedtherewith, arranged to receive light that passes through said reflectivesurface.

In a preferred embodiment, at least a Hartmann mask of a Hartmannwavefront sensor is integrated with the mirror. If the mirror comprisesa mirror substrate with the reflective surface applied thereto, then theHartmann mask may comprise an array of apertures formed in the mirrorsubstrate. Alternatively, if the mirror comprises a substantiallytransparent mirror substrate, then the Hartmann mask may be formed in adistinct layer within the mirror structure, for example one applied to asurface of the mirror substrate.

Preferably, the Hartmann mask is covered by a planarisation layer towhich said reflective surface is applied. A planarisation layer mayprovide an optically flat and polished surface on which to apply thereflective surface of the mirror.

In a further preferred embodiment, the at least one component of anoptical sensor is a microlens array of a Shack-Hartmann wavefront sensorintegrated with the mirror. The microlens array may be covered by aplanarisation layer to provide an optically flat polished surface forthe reflective surface.

In a further preferred embodiment, the at least one component of anoptical sensor is an optical detector. The optical detector may take oneof a number of different forms according to what properties of theincident light are to be detected. For example, the optical detector maycomprise a charge-coupled device (CCD) or position sensing detector(PSD). Such an optical detector may be integrated with either a planemirror or with a deformable mirror that has an active layer ofdeformable material coupled to a passive substrate.

Preferably the optical detector is provided to detect the wavefrontslope of the incident light at one or more points across the aperture ofthe mirror.

In the preferred Hartmann mask embodiments, the mirror may be a fixedprofile mirror or it may be a deformable mirror. In a preferredembodiment, the mirror is a self-deforming mirror comprising an activelayer of deformable material coupled to a substantially transparentpassive substrate and wherein the Hartmann mask comprises an array ofapertures formed in the active layer of deformable material. Of course,if the active layer is formed using a substantially transparentdeformable material, such as a known form of single crystalpiezo-electric material, then the Hartmann mask may be formed in anotherway, for example by an array of small apertures in a common electrodelayer of the deformable mirror.

Preferably, the self-deforming mirror is supported by means of one ormore passive flexible support elements coupled to a housing and whereinan optical detector is provided within the housing to detect thewavefront slope of the incident light at one or more points across theaperture of the mirror.

The optical sensor in preferred embodiments of the present invention maycomprise a Charge Coupled Device (CCD) array, a Position SensitiveDetector (PSD) or array of PSDs, a quad detector or array of quaddetectors, or a Hartmann wavefront sensor.

From a second aspect, the present invention resides in a laser mirrorcomprising a mirror structure according to the first aspect, above. Sucha device may be used as the main component in a simplified intra-cavityadaptive optics system, removing the need for a separate sensor andwavefront corrector. Because the sensor is integrated behind a mirrorsurface, only small amounts of light will reach the sensor. Whileintegration of the sensor behind a mirror surface will excludephoton-limited applications such as Astronomy, this type of device isideal for use with laser systems where interrogation of the wavefront orbeam profile often requires some form of attenuation to ensure thatexcess laser power does not damage the optical sensor.

A mirror structure incorporating at least one component of a Hartmann ora Shack-Hartmann wavefront sensor may be used with a laser cavity ineither a closed loop or an open loop configuration.

Preferred embodiments of the present invention will now be described inmore detail, by way of example only, with reference to the accompanyingdrawings of which:

FIG. 1 is a diagram representing a sectional view through a knownShack-Hartmann sensor;

FIG. 2 is a diagram representing a simple known laser cavity;

FIG. 3 is a diagram representing a known intra-cavity laser adaptiveoptics system;

FIG. 4 provides a sectional view and a plan view of an image sensorintegrated with a laser mirror, according to a preferred embodiment ofthe present invention;

FIG. 5 provides a sectional view and a plan view of an image sensorintegrated with a deformable mirror, according to a preferred embodimentof the present invention;

FIG. 6 provides a sectional view and a plan view of a mirror structurewith an integrated Hartmann mask, according to a preferred embodiment ofthe present invention;

FIG. 7 provides a sectional view and a plan view of a mirror structurewith an integrated Shack-Hartmann sensor, according to a preferredembodiment of the present invention; and

FIG. 8 provides a sectional view through a mirror structure comprising aperipherally-supported deformable mirror with an integrated Hartmannmask, according to a preferred embodiment of the present invention.

Preferred embodiments of the present invention have been devised toincorporate one of several known types of optical wavefront sensor in amirror structure giving particular advantages in simplifying adaptiveoptics systems, in particular for use in laser cavities. However, by wayof introduction, a known type of wavefront sensor—the Shack-Hartmannsensor—suitable for use in preferred embodiments of the presentinvention, will firstly be described with reference to FIG. 1.

Referring to FIG. 1, a sectional view is provided through a conventionalShack-Hartmann sensor 10 comprising an array of microlenses 12 that eachsample an incident wavefront 14 over a sub-aperture of the sensor 10,and an optical sensor 16, comprising an array (typically square) ofindividual CCDs or PSDs that each detect the position of a focussed spotof light 18 in the respective sub-aperture. Position data from the imagesensor 16 provides a set of measurements of average wavefront slope 20across each sub-aperture as represented by the displacement 22 of therespective focussed spot of light 18 from the axis 24 of the focussingmicrolens 12.

In order to provide an accurate representation of the total wavefront14, the size of each sub-aperture, and hence of each microlens 12, ismade small enough to capture the highest order aberration that needs tobe corrected for. However, the complexity of subsequent data processingincreases rapidly with the number of microlenses 12 in the array,reducing the overall rate of response of the sensor 10 to changes in thewavefront 14.

Where the application is not photon-limited, it is known to replace thearray of microlenses 12 with a so-called Hartmann mask. A Hartmann maskis generally a mechanical mask with a square array of apertures. Whilethe principle of operation of a sensor incorporating a Hartmann mask isthe same as for the conventional Shack-Hartmann sensor 10, the Hartmannmask will obstruct more of the incident light than a microlens array,limiting its use to applications in which the light levels are going tobe sufficiently high to be detectable by the image sensor 16 afterpassing through the Hartmann mask.

A simple known laser cavity structure will now be described briefly withreference to FIG. 2 as one example of a potential application ofHartmann wavefront sensors. Features of this simple laser cavity will bereferenced later when discussing applications of the present invention.

Referring to FIG. 2, a simple laser cavity 25 is bounded by a firstlaser mirror 26 and a second, partially reflective laser mirror 27.Within the cavity a laser beam 28 is generated by a lasing medium 29 andtravels between the first and second mirrors 26, 27 before being outputfrom the partially reflective second mirror 27.

It is known to use Shack-Hartmann sensors 10 directly as diagnostictools for deriving wavefront and intensity information for a laser, andhence the intensity profile, as one measure of the quality of a laserbeam. In particular, it is known to use either or both of a conventionalCCD array and a Shack-Hartmann sensor in conjunction with a wavefrontcorrector or deformable mirror as part of an Adaptive Optics System, forexample in an intra-cavity laser adaptive optics (AO) arrangement aswill now be described in outline with reference to FIG. 3.

Referring to FIG. 3, a representation is provided of a laser cavity 30that is bounded by a first laser mirror 32 and a second, partiallyreflective output mirror 34. A laser beam 36, generated within a lasingmedium 38, travels between the first laser mirror 32 and the second,partially reflective output mirror 34 by way of a third, fixed mirror 40and a deformable mirror 42. Light from the laser beam 36 is sampled by abeam splitter 44 placed in the path of the laser beam 36 between thedeformable mirror 42 and the output mirror 34 to direct a portion oflight 46 towards a wavefront sensor 48, for example a Shack-Hartmannsensor 10 as described above with reference to FIG. 1, where thewavefront profile of the laser beam 36 may be detected. The wavefrontsensor 48 outputs data to a controller 50 which determines, on the basisof those data, an appropriate deformation profile to apply to thedeformable mirror 42 so as to make corresponding adjustments to thelaser beam wavefront as is reflects from the deformable mirror 42 withinthe laser cavity 30.

A Shack-Hartmann sensor 10 may be used as a wavefront sensor 48 withinthe laser cavity 30 in either a closed loop or an open loopconfiguration. The distortions induced in a laser cavity are generallythermal in nature and so there will be no instantaneous response due tomodifications applied by a wavefront corrector such as the deformablemirror 42. For this reason, the controller 50 is generally arranged toimplement an adaptive control system based around an iterative searchalgorithm. In a closed loop configuration such as that shown in FIG. 3,the wavefront sensor 48 is placed after the wavefront correctingdeformable mirror 42 so that it will detect any residual wavefronterrors. An iterative control algorithm may be implemented by thecontroller 50 so that the system converges on an optimal level ofcorrection for the laser beam 36. This provides for more accuratewavefront correction than is possible in an open loop system in whichthe wavefront sensor precedes the wavefront corrector and there is nofeedback loop, but its response will be faster than in a closed loopsystem.

Preferred embodiments of the present invention will now be describedwhich enable a simplification of the adaptive optics intra-cavityarrangement of FIG. 3, amongst other advantages. A first preferredembodiment will be now be described with reference to FIG. 4.

Referring initially to FIG. 4 a, a sectional view is provided through amirror structure 60 comprising a fixed mirror having a reflectivesurface 62 that is arranged to allow a small amount of light to passthrough it, and a CCD or CMOS active pixel sensor array 64 integratedwithin the mirror structure 60. The integrated sensor array 64 isarranged within the structure 60 such that light that passes through thereflective surface 62 may be detected by the sensor array 64. Referringadditionally to FIG. 4 b, a plan view is provided of the same mirrorstructure 60 from a view point above the reflective surface 62.

Preferably, the mirror structure 60 comprises a thin layer (of the orderof 300 μm) of silicon 66 on which the CCD or CMOS active pixel sensorarray 64 is fabricated using conventional fabrication techniques. Thesize of the sensor array 64 is chosen to be wider than the intendedoptical aperture 65 which, in the case of an intra-cavity laser beam, islikely to be significantly less wide than the diameter of the reflectivesurface 62 and most likely confined to the central region of thatsurface 62. A planarisation and/or passivation layer 68, preferably ofsilicon dioxide or silicon nitride, or one layer of each, is applied tocover the sensor array 64 to a thickness sufficient to enable anoptically flat polished surface to be provided above the sensor array64, suitable for the application of the reflective surface 62. Apassivation layer, in particular, may be required to ensure that anyelectrically conducting tracks on the surface of the silicon layer 66are not shorted, in particular if the reflective surface 62 is providedby a coating of gold, for example. A planarisation layer may be requiredfor example if the sensor array fabrication process leaves behind smallsurface features, for example tracking or exposed circuitry, which wouldlead to undesirable “print-through” on the reflective surface 62. Aplanarisation layer would provide for a high quality, optically flatreflective surface 62.

The planarisation/passivation layers 68 are required to be at leastpartially transparent to light at the operating wavelength for themirror, e.g. for a laser mirror, so that light may reach and bedetectable by the sensor array 64.

Preferably the silicon layer 66 is bonded to a rigid and supportivecarrier layer 70, preferably made from un-processed silicon or siliconcarbide so as to have a similar expansion coefficient to the siliconlayer 66 and, in the case of silicon carbide, to provide a highlythermally conductive layer to assist in the efficient removal of heatfrom the mirror structure 60 when applied to laser cavities. Because thesilicon layer 66 is likely to be thin (e.g. 300 μm), it may bebeneficial during assembly of the structure 60 to bond the silicon layer66 to the carrier layer 70 before the planarisation/passivation layer 68is polished flat, to provide additional support.

Preferably, the electrically conducting tracks to the sensors of thearray 64 extend to the periphery of the silicon layer 66 and terminatewith serial output pads 72 provided around the periphery of the mirrorstructure 60. Interconnection to the peripheral serial output pads 72may be achieved in a number of known ways, for example using aflexi-circuit of a type described in published international patentapplication WO 2005/124425, an elastomeric connector, wire bonding orsoldering.

The level of attenuation to be introduced by the reflective surfacecoating 62 of the mirror is determined by a requirement to match, in agiven application, the intensity of light transmitted to the sensorarray 64 to the sensitivity of the sensors in the array 64. Thereflective surface 62 preferably comprises a coating of gold, enhancedgold or a multi-layer dielectric coating.

In an alternative arrangement of the mirror structure 60, the sensorarray 64 may be integrated/bonded to a deeper layer within the mirrorstructure 60, or even to the lowest surface of the mirror structure, ifthe intermediate layer or layers of substrate material are sufficientlytransparent.

The mirror structure 60 with the integrated sensor array 64, describedabove, may be used in any number of known configurations. For example,in the laser cavity 30 described above with reference to FIG. 3, themirror structure 60 may replace one or both of the first or second lasermirrors 32 or 40. This would simplify the laser cavity 30, removing theneed for the beamsplitter 44 and separate sensor 48. If two such mirrorstructures 60 are incorporated in the laser cavity 30, each maycontribute different measures of laser beam quality to an adaptiveoptics control system.

The mirror structure 60 may be used in conjunction with an iterativesearch algorithm to control a deformable mirror for intra-cavityadaptive optics. One such algorithm is a hill-climbing algorithm whichmodifies the deformable mirror to maximise the optical power within aspecific ‘software aperture’ on the sensor array 64. This type ofalgorithm is particularly suited to intra-cavity adaptive optics wherethe slow response of the cavity means that, after an adjustment of thedeformable mirror, the cavity needs to be left to stabilise before anyfurther adjustments are made.

In a second preferred embodiment of the present invention, a mirrorstructure similar to the structure 60 described above, excluding thecarrier layer 70, may form the basis of a deformable mirror structure,as will now be described with reference to FIG. 5.

Referring to FIG. 5, a sectional view is provided through aself-deforming mirror structure 80 comprising a thin (of the order of300 μm) passive substrate layer 82, preferably of silicon andcorresponding to the silicon layer 66 in FIG. 4, bonded to an active PZTlayer 84. A sensor array 86 has been fabricated on the silicon layer 82by conventional fabrication techniques, and a planarisation and/orpassivation layer 88 has been applied to cover the sensor array 86 andany associated tracks to provide an optically flat surface for theapplication of an at least partially transmissive reflective surface 90.Sensor output connection pads 92 are provided around the periphery ofthe deformable mirror structure 80 for interconnection via a cable 93with the sensor array 86.

The active PZT layer 84 is provided on one face with a thin continuouselectrode layer (not shown in FIG. 5), sandwiched between the siliconlayer 82 and the PZT layer 84, and on the other face with a patternedarray of electrodes 94 to enable selected regions of the PZT layer 84 tobe energised and thus deformed. Preferably a flexi-circuit 96 of thetype referenced above may be connected to the electrodes 94 and aninterconnect cable 98 may be provided to enable voltages to be suppliedselectively to the electrodes 94. The self-deforming parts (82 to 96 inFIG. 5) of the mirror structure 80 are supported from below around theirperiphery by means of one or more compliant support elements 100 mountedon a rigid base 102.

Whereas, in the mirror structure 80, the silicon layer 82 is bonded to apatterned PZT layer 84 to form a deformable mirror, it may alternativelybe bonded to an array of actuators, for example piezo-tubes, PZT stacksor MEMS actuators to form a zonal deformable mirror. The silicon layer82 may alternatively be added to a true bimorph deformable mirrorstructure, or form the top layer of a symmetric bimorph structure.

As an alternative to PZT, the layer of active material 84 may comprise alayer of a single crystal piezo-electric material of which knownexamples are substantially transparent to light, or a layer of anelectrostrictive material.

The mirror structure 80 may conveniently be retro-fitted into anexisting laser cavity, for example replacing the first laser mirror 26in the simple laser cavity 25 of FIG. 2 to enable adaptive optics to beadded without increasing the complexity of the cavity. The usefulness ofa deformable mirror in a simple laser cavity 25 is dependent upon thewidth of the laser beam 28 within the cavity 25. If the beam width istoo small then only the very lowest order distortion (e.g. spherical)would be correctable by the mirror structure 80. However, such a mirrorstructure 80 may still prove very useful; the main optical distortionthat will be present in a cavity will be driven by thermal lensing.

In a third preferred embodiment of the present invention, a mirrorstructure will now be described with reference to FIG. 6 in which aHartmann mask is integrated with a laser mirror.

Referring initially to FIG. 6 a, a sectional view is provided through amirror structure 110 comprising a transparent mirror substrate 112, madefrom glass for example, a reflective surface 114 that is arranged toallow a small amount of light to pass through it, a Hartmann mask 116disposed between the reflective surface 114 and the mirror substrate 112and a sensor 118 fabricated on a layer of silicon 120 or othersemi-conductor material, arranged to receive light transmitted throughthe reflective surface 114, the apertures of the Hartmann mask 116 andthe mirror substrate 112. Referring additionally to FIG. 6 b, a planview is provided of the same mirror structure 110 viewed from a positionabove the reflective surface 114.

The Hartmann mask 116 may comprise either a pre-formed Hartmann maskthat is bonded to the mirror substrate 112 or a Hartmann mask formed byphotolithography directly onto the surface of the mirror substrate 112.

A planarisation layer 122 is provided to cover the Hartmann mask 116 andto provide an optically flat and transparent polished surface onto whichthe reflective surface 114, comprising a thin deposit of gold forexample, is formed. Alternatively, the Hartmann Mask 116 may be bondedbetween two glass substrates.

The depth of the mirror substrate 112, and hence the separation of thesensor 118 from the Hartmann mask 116, is selected according to themaximum wavefront slope that needs to be measured and the resolution ofthe sensor 118. The sensor 118 may comprise a conventional CCD or anarray of PSDs as described above, covered if necessary by aplanarisation layer (not shown in FIG. 6) to give a flat contact surfacewith the mirror substrate 112, and may be mounted on the back of themirror substrate 112 in such a way that it may be replaced if necessary.Interconnection to the sensor is provided by means if connection pads124 provided around the periphery of the silicon layer 120.

For applications that involve use of the mirror structure 110 in a harshenvironment, the sensor 118 may be hermetically bonded to the back ofthe mirror substrate 112.

In view of the high level of optical attenuation provided by thecombination of a reflective surface coating 114 and a Hartmann Mask 116,the mirror structure 110 according to this third preferred embodiment islikely to be used in a high power laser system.

In a fourth preferred embodiment of the present invention, a mirrorstructure is provided in which a microlens array and a sensor areintegrated with a laser mirror to provide an integrated Shack-Hartmannsensor. The mirror structure according to this fourth preferredembodiment will now be described with reference to FIG. 7.

Referring initially to FIG. 7 a, a sectional view is provided through amirror structure 130 comprising a transparent mirror substrate 132, madefrom glass for example, a reflective surface 134 that is arranged toallow a small amount of light to pass through it, a microlens array 136disposed between the reflective surface 134 and the mirror substrate 132and an optical sensor 138 arranged to receive light transmitted throughthe reflective surface 134, the microlens array 136 and the mirrorsubstrate 132. Referring additionally to FIG. 7 b, a plan view isprovided of the same mirror structure 130 viewed from a position abovethe reflective surface 134.

In common with the sensors used in preferred embodiments describedabove, the sensor 138 comprises an CCD or an array of PSDs fabricated ona layer of silicon 140 or other semi-conductor material, covered ifnecessary with a planarisation layer 142 to provide a flat surface forbonding to the mirror substrate 132. Connection pads 139 are provided,as above, for interconnection with the sensor 138. A furtherplanarisation layer 144 is formed to cover the microlens array 136 forthe purpose of providing an optically flat polished surface ready toreceive a layer of gold, for example, to form the reflective surface134. However, in order for the microlenses in the array 136 to functionas lenses, the microlenses are formed using a material with a differentrefractive index to the material forming the planarisation layer 144.

In common with the third preferred embodiment described above, the depthof the transparent mirror substrate 132 is selected according to themaximum wavefront slope to be measured and the resolution of the sensor138.

In a fifth preferred embodiment of the present invention, a mirrorstructure will now be described with reference to FIG. 8 in which aHartmann mask is integrated with a self-deforming mirror.

Referring to FIG. 8, a sectional view of is provided through a mirrorstructure 150 in which an otherwise conventional self-deforming mirror,having an active layer 152 of deformable material such as PZT bonded toan optically transparent passive substrate 154 made from glass forexample, with an intermediate common electrode layer (not shown in FIG.8), has been modified by drilling an array of apertures 156 through thenon-transparent layers, in particular the active PZT layer 152, to forma Hartmann mask. However, if a transparent single crystal piezo-electricmaterial were to be used for the active layer 152, then the Hartmannmask may be implemented in a different way and may comprise an array orapertures formed only in the common electrode layer for example.

A partially transmissive reflective surface 158, for example of gold, iscarried by the passive substrate 154, if necessary formed on anoptically flat polished surface of a thin, optically transparentplanarisation layer (not shown in FIG. 8) applied to cover the passivesubstrate 154. A partitioned electrode layer 160 is provided to enableselected regions of the active layer 152 to be energised via aflexi-circuit 162 of a type referenced above. An interconnect cable 164connects to the flexi-circuit.

The deformable mirror, comprising the components 152 to 164, issupported preferably from below by means of a number of discrete passiveflexible support elements 166 disposed around its periphery or,alternatively, by a continuous annular flexible support element 166. Thesupport elements 166 are mounted on an annular section 170 of a rigidhousing that comprises the annular section 170 fixed to a base plate172. The rigid housing 170, 172 encloses a cavity 174 beneath thesupported deformable mirror components.

An optical sensor 176 is provided within the rigid housing 170, 172,conveniently mounted within the cavity 174 on the base plate 172,comprising an array of discrete quad detectors or an array of CCDsfabricated on a wafer of silicon 178 with each discrete detector centredbelow the position of a respective aperture 156 of the Hartmann mask.Connection to the detectors of the sensor array 176 is preferably bymeans of a flexi-circuit and a further interconnect cable 180.

Light from an incident wavefront 155 passes through the apertures 156 ofthe Hartmann mask and through the cavity 174 to be detected by thesensor 176. The distance h separating the apertures 156 of the Hartmannmask and the sensor 176 is chosen according to the maximum wavefrontslope that needs to be measured and the resolution of the sensor 176.The displacement of the centroid of a spot of light at the sensor 176for an incident wavefront of slope 8 is given byΔx=θh

The size of the apertures 156 is chosen to be large enough to letthrough enough of the incident light to be detectable by the sensor 176but to be small enough so that “print-through” due to the apertures 156is negligible at the mirror reflective surface 158. Furthermore, becausethe apertures 156 for the Hartmann Mask are being created in the activePZT layer 152, there will be a limit on the number of apertures 156(Hartmann “windows”) and hence sensor array size that can be used. Asmore and more Hartmann windows are added, the amount of PZT remaining inthe active layer 152 to generate the required mirror curvatures will bereduced, restricting the voltage sensitivity of the device. The positionof each aperture 156 in the active layer 152 preferably coincides with agap between electrodes in the electrode layer 160 to minimise the numberof apertures that need to be formed in the electrodes. Correspondinglypositioned apertures are made in the flexi-circuit 162.

In this fifth preferred embodiment shown in FIG. 8, it has been foundthat deformation of the active PZT layer 152 does not significantlyaffect the displacement of the centroids of light spots at the sensorarray 176.

Whereas the mirror structure 150 in this fifth preferred embodiment isshown comprising a deformable mirror with a single active PZT layer 152,a deformable mirror may be integrated with the structure 150 that usesother materials or is of a different type, for example a true bimorph ora symmetrical bimorph type of deformable mirror, or one of a type thatis deformed using stacked actuators, PZT tubes, or MEMS devices.

The mirror structure 150 of this fifth preferred embodiment may be usedto replace the first laser mirror 26 in the simple laser cavity 25 ofFIG. 2. In such an application, while there may be a limit on the numberof individual Hartmann windows that can be added, this may not be anissue for intra-cavity wavefront correction where the main aberrationthat requires correction is the thermal lensing of the laser rod 28.Depending on the geometry, the main distortions requiring correctionwill be either spherical or astigmatic. Spherical aberrations will becircularly symmetrical, and so may be determined using just one line ofHartmann windows/apertures 156 along a radius of the mirror. Theastigmatic case may be determined with two orthogonal lines of Hartmannwindows/apertures 156, each along a radius of the mirror.

The mirror structure 150 may be used extra-cavity but the control systemwould be based either on an iterative search algorithm system or itwould be configured for open loop control. In a further refinement, themirror structure 150 may be used in a distributed control configurationwhere the control for a particular element of the deformable mirror iscalculated based only upon the outputs of sensors in the sensor array176 that are closest to the axis of the mirror element.

The mirror structures in preferred embodiments of the present invention,described above, have provided different examples of both fixed anddeformable mirrors each having at least a part of an optical wavefrontsensor integrated with them to enable light passing through thereflective surfaces of the mirrors to be detected for the purpose ofwavefront correction. Those parts of a wavefront sensor have included atleast a Hartmann Mask for a Hartmann sensor or the microlens array for aShack-Hartmann sensor. Preferred positions of those parts within themirror structures have been discussed but preferred mirror structuresaccording to the present invention would not be limited to thosepositions. Furthermore, preferred sensor configurations have also beendiscussed, but preferred mirror structures according to the presentinvention are not limited to those configurations and alternativeconfigurations are included within the scope of the present invention,as would be apparent to a person of ordinary skill in this field.

Whereas laser applications have been discussed, in particular, it willbe clear that mirror structures according to the present invention maybe applied to any field where the incident light levels are sufficientto enable wavefront measurement using those mirror structures.

The invention claimed is:
 1. A mirror structure, comprising a mirrorhaving a reflective surface arranged to allow a portion of incidentlight to pass therethrough and wherein the mirror has at least onecomponent of an optical sensor integrated therewith, arranged to receivelight that passes through said reflective surface.
 2. The mirrorstructure according to claim 1, wherein said at least one component ofan optical sensor is a Hartmann mask of an optical wavefront sensor. 3.The mirror structure according to claim 2, wherein the mirror furthercomprises a mirror substrate with said reflective surface appliedthereto and wherein the Hartmann mask comprises an array of aperturesformed in the mirror substrate.
 4. The mirror structure according toclaim 2, wherein the mirror further comprises a substantiallytransparent mirror substrate and wherein the Hartmann mask is formed asa distinct layer within the mirror structure.
 5. The mirror structureaccording to claim 4, wherein the Hartmann mask is formed in a layerapplied to a surface of the mirror substrate.
 6. The mirror structureaccording to claim 1, wherein said at least one component of an opticalsensor is a Hartmann mask of an optical wavefront sensor and wherein themirror further comprises a mirror substrate with said reflective surfaceapplied thereto and wherein the Hartmann mask comprises an array ofapertures formed in the mirror substrate and wherein the Hartmann maskis covered by a substantially transparent planarisation layer to whichsaid reflective surface is applied.
 7. The mirror structure according toclaim 1, wherein said at least one component of an optical sensor is amicrolens array of a Shack-Hartmann wavefront sensor.
 8. The mirrorstructure according to claim 7, wherein the microlens array is coveredby a substantially transparent planarisation layer and the reflectivesurface is applied thereto.
 9. The mirror structure according to claim1, wherein said at least one component of an optical sensor is anoptical detector.
 10. The mirror structure according to claim. 9,wherein the mirror is a self-deforming mirror having an active layer ofdeformable material coupled to a passive substrate.
 11. The mirrorstructure according to claim 9, wherein said optical detector isarranged to detect the wavefront slope of said incident light at one ormore points across the aperture of the mirror.
 12. The mirror structureaccording to claim 2, wherein the mirror is a self-deforming mirrorhaving an active layer of deformable material coupled to a substantiallytransparent passive substrate and wherein the Hartmann mask comprises anarray of apertures formed in the active layer of deformable material.13. The mirror structure according to claim 12, wherein theself-deforming mirror is supported by means of one or more passiveflexible support elements coupled to a housing and wherein an opticaldetector is provided within the housing to detect the wavefront slope ofsaid incident light at one or more points corresponding to the positionsof said apertures.
 14. A laser mirror comprising a mirror structure, themirror structure comprising a mirror having a reflective surfacearranged to allow a portion of incident light to pass therethrough andwherein the mirror has at least one component of an optical sensorintegrated therewith, arranged to receive light that passes through saidreflective surface.